System for assessing lachrymal fluid content of a sample pad

ABSTRACT

A system rapidly and accurately assesses the lachrymal fluid content in a person&#39;s eye which minimizes patient discomfort. The preferred system includes a lachrymal fluid sample device and an assessment device. The preferred sampling device includes a lachrymal fluid absorbent sample pad and a non-absorbent handle. The preferred assessment device includes a receiver for receiving a lachrymal fluid sample pad and includes a control circuit coupled with the receiver for determining the electrical capacitance of the pad which capacitance varies in accordance with variations in lachrymal fluid content, and for producing an output correlated with the lachrymal fluid quantity in the pad.

This application is a continuation-in-part application of Ser. No.07/393,223 filed Aug. 14, 1989 now U.S. Pat. No. 4,994,751.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns an accurate and rapid system forassessing lachrymal fluid content in a person's eye and which alsominimizes patient discomfort. More particularly, the invention concernsa sampling device for taking a representative lachrymal fluid samplefrom a person's eye, and an assessment device which determines anelectrical parameter, preferably capacitance, which varies in accordancewith the lachrymal fluid content of the sampling device and produces anoutput correlated therewith, such being thereby correlated with thelachrymal fluid content of the person's eye from which the sample wastaken.

2. Description of the Prior Art

The lachrymal fluid content of a person's eye has a bearing on generaleye health, and in particular, is a factor in whether a person can wearcontact lenses. That is to say, if a person's eyes produce insufficientlachrymal fluid, that is tears, then contact lenses are contra-indicatedbecause the potential for irritation and discomfort.

Those skilled in the art are familiar with the so-called Schirmer testfor assessing the lachrymal fluid content of a person's eye. TheSchirmer test developed in 1909, comprises an elongated, fluid absorbentsample member made of filter paper. One end of the member is insertedunder the lower eye lid of the eye being checked with the other endprotruding. The sample member is retained in the eye for five minutes inorder to allow the member to absorb a sufficient sample amount. Thedistance the lachrymal fluid migrates or "wicks" along the member towardthe exposed end is supposed to provide an indication of the lachrymalcontent of the person's eye. In reality, it performs more as a test ofthe eyes ability to produce lachrymal in response to the irritation andstress induced by the presence of the sample member.

As those skilled in the art will appreciate, the Schirmer test is notconsidered accurate because the relatively long term presence of thesampling member in a person's eye causes irritation and, in response,excess fluid production which distorts the results of the test. Becausethe test relies on wicking action, it also presents some inherentinaccuracies.

SUMMARY OF THE INVENTION

The present invention solves the prior art problems as outlined above.That is to say, the invention hereof provides an accurate and rapidsystem for assessing lachrymal fluid content in a person's eye in amanner which minimizes patient discomfort.

Broadly speaking, the system hereof includes a sampling device and anassessment device. The sampling device includes a sample pad which isabsorbent of lachrymal fluid at a predetermined known rate, and a handleportion which is non-absorbent of lachrymal fluid to prevent wickingtherealong and used for placing the sample pad in substantially flushengagement with the patient's eye surface.

The preferred assessment device includes a receiver for receiving alachrymal fluid sample pad such as that described above and anelectrical circuit coupled with the receiver for determining anelectrical parameter, preferably capacitance of the pad, and responsivethereto for producing an output correlated with the lachrymal fluidcontent of the sample pad and thereby correlated with the lachrymalfluid amount present in the person's eye from which the sample wastaken.

In preferred forms, the receiver includes a pair of electrodes whichdefine a sample pad receiving space therebetween. The preferredelectrical circuit includes a microprocessor with an associated memoryfor storing correlation data which is used to correlate the output ofthe circuit with the electrical parameter. Other preferred aspects ofthe invention are discussed further hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the preferred lachrymal fluid samplingdevice;

FIG. 2 is a perspective view of the preferred assessment device showingthe shiftable electrode in the retracted position;

FIG. 3 is an electrical schematic diagram of the capacitance determiningcircuit of the assessment device of FIG. 1;

FIG. 4 is an electrical schematic diagram of the signal processingcircuit of the assessment device;

FIG. 5 is an electrical schematic diagram of the display circuit of theassessment device; and

FIG. 6 is a computer program flowchart of the operating program foroperating the assessment device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to FIG. 1 preferred lachrymal fluid sampling device10 includes disk-shaped sample pad 12 and elongated handle 14, one endof which is coupled with an edge of sample pad 12 to present thestructure illustrated. Sample pad 12 is preferably composed of materialabsorbent of lachrymal fluid at a predetermined known rate. Suchmaterials include polyvinylformal presenting a pore size in the rangebetween about 30 and 150 microns and preferably about 60 microns or,less preferred, Whatman 3 filter paper. Polyvinylformal is advantageousbecause it is FDA approved as safe for contact with the surface of aperson's eye and specifically with the sclera thereof. Sample pad 12preferably presents a circular, relatively flat, disk-shapedconfiguration presenting a known, predetermined surface area preferablyselected from between about 25 to 30 square millimeters in the area.Another embodiment of sample pad 12 presents a frusto-circular shape inplan view about 6 millimeters long, with a curvature diameter of about 5millimeters and a thickness of about 1 millimeter.

Handle 14 is preferably composed of relatively thin, synthetic resinmaterial such as mylar about 25 mm. long, 5 mm. wide and 5 mils thick.This construction presents a handle which is nonabsorbent of lachrymalfluid in order to prevent fluid migration or wicking therealong. Suchwicking has led to inaccuracies in the prior art. Preferred samplingdevice 10 also includes a removable, U-shaped, milar cover disposed oversample pad 12 for maintaining the sterility thereof before use.

FIG. 2 presents a perspective view of preferred assessment device 20which is used to determine an electrical parameter, preferablycapacitance, which varies in accordance with variations in the quantityof lachrymal fluid present in sample pad 12. As illustrated, assessmentdevice 20 includes housing 22, axially shiftable electrode 24, and fixedelectrode 26. In another preferred embodiment, the electrodes areadjustable but retain a fixed setting which avoids variations in the gapbetween electrodes and thereby produces more consistent results.

Housing 22 is preferably composed of synthetic resin material andincludes base 28, shiftable electrode support member 30, fixed electrodesupport member 32, and upstanding intermediate support member 34.

Base 28 encloses control circuit 36 (FIGS. 3-5) and includes shiftswitch 38 for electrically actuating shiftable electrode 24 leftwardlyas shown in FIG. 2, zero switch 40 which is used to zero circuit 36 asexplained further hereinbelow, and liquid crystal display 42.

Shiftable electrode 24 presents a generally cylindrical configurationand includes central section 44 presenting stop shoulder 46, smallerdiameter electrode section 48 extending rightwardly (as viewed in FIG.2) from stop shoulder 46 and through intermediate support 34 towardfixed electrode 26, and outboard section 50 extending leftwardly fromcentral section 44. As illustrated, sections 44, 48, and 50 are axiallyaligned. Housing support member 30 encloses a conventional, +10 v.d.c.,solenoid coil (not shown) which surrounds that portion of shiftableelectrode 24 therein and is electrically connected with shift switch 38for electromagnetic shifting of electrode 24 to the retracted positionas shown in FIG. 2. Housing support member 30 also encloses conventionalbiasing spring (not shown) which abuts the leftward shoulder (not shown)of central section 44 in order to bias electrode 24 rightwardly to theengaged position. FIG. 2 illustrates shiftable electrode 24 in theshifted or retracted position as actuated by activation of shift switch38.

Fixed electrode 26 is threadably coupled with support member 32 to allowaxial adjustment thereof. Electrodes 24, 26 define sample pad receivingspace 52 therebetween. When deenergized, the biasing spring shiftselectrode 24 rightwardly until stop shoulder 46 abuts the leftward faceof intermediate support 34, thus assuring return of electrode 24 to thesame position each time in order to present precisely controlled anduniform receiving space 52.

In the unretracted position, space 52 is narrow enough to slightlycompress a sample pad received therein in order to hold it in place. Thedimension of receiving space 52 can be adjusted very precisely by meansof the threaded connection of fixed electrode 26 with housing supportmember 32.

In the use of device 10, handle 14 is grasped with the preferred coverin place and first inserted in receiving space 52 between electrodes24,26 in order to initially "zero" assessment device 20 before taking asample from the patient's eye. The preferred cover over sample pad 12prevents contamination thereof by electrodes 24,26. After assessmentdevice 20 is zeroed for the particular sampling device 10, the preferredcover remains in place in receiving space 52 while sampling device 10without the cover is then removed for taking a sample.

One face of sample pad 12 is then placed in flush engagement with thesclera of the patient's eye for a predetermined sample time, preferablythree to five seconds. The sample time is chosen to be long enough toabsorb a representative lachrymal fluid sample but short enough so thatthe pad is removed before the patient physiologically responds withinincreased production of lachrymal fluid which has been a problem in theprior art. The preferred sample time of about three to five secondscompares to five minutes, for example, in the prior art. The shortsampling time of the present invention can be accurately counted by theuser and also minimizes patient discomfort.

As those skilled in the art will appreciate, the amount of lachrymalfluid present in a typical patient eye is extremely small, in the rangeof 5 microliters, and the sample quantity absorbed by sample pad 12 iseven smaller, in the range of 0.5 microliters. Accordingly, and in orderto provide uniform and repeatable results, it is preferred that thedimensions of sample pad 12 be precisely controlled during themanufacture to provide uniformity in dimensions and uniformity in thestructure of the sample pad material itself. It is also preferred thatthe sample time also be precisely controlled so that the only variablein the sampling process is the lachrymal fluid present in the patient'seye. As explained further hereinbelow, the preferred assessment device20 allows compensation for relative humidity and slight variations indifferent sampling devices 10 by the zeroing or calibrating procedure.

After taking a lachrymal sample, the user holds device 10 by graspinghandle 14 in order to place sample pad 12 in space 52 and between thedistal portions of the preferred cover which remained in place duringthe sampling. Shift switch 38 is then activated to retract electrode 24.After pad 12 is in placed in space 52, switch 38 is released andelectrode 24 returns to the engaged position to hold sample pad 12against fixed electrode 26. Sample handle 14 can then be released.

To ensure even greater accuracy, electrode 24 can be configured alsonon-shiftable which presents receiving space 52 as a fixed gap. In thisway, variations are eliminated in the return of electrode 24 to itsoriginal position. In this configuration, electrodes 24,26 present apreferred diameter of 5/16 inch and define receiving space 52 such thata gap of about 0.045 inches between the distal portions of the samplepad cover.

Control circuit 36 includes capacitance circuit 54 (FIG. 3), signalprocessing circuit 56 (FIG. 4), and display circuit 58 (FIG. 5). Controlcircuit 36 also includes a conventional power supply well known to thoseskilled in the art for receiving operating power from a conventional 120v.a.c. outlet or batteries in order to provide required operatingvoltages at +10 and +5 v.d.c., and to supply operating power thesolenoid coil for shifting electrode 24.

Capacitance circuit 54 (FIG. 3) determines the capacitance betweenelectrodes 24, 26 which represented in FIG. 3 as capacitor 60. Ingeneral, circuit 54 determines the capacitance, which varies inaccordance with the amount of lachrymal fluid present in a sample, andprovides a capacitance signal at output terminal 62. This signal is inthe form of a square wave the frequency of which varies in accordancewith the capacitance of capacitor 60.

As those skilled in the art will appreciate, a sample pad with lachrymalfluid present therein presents other parameters, such as resistance,which could be used for determining the amount of lachrymal fluidpresent in the sample pad and converted into a suitable and useablesignal. In the preferred environment, however, determination of samplepad capacitance is preferred as providing the most accurate andrepeatably parameter.

Capacitance circuit 54 broadly includes positive constant current source64, negative constant current source 66, switching circuit 68, isolationamplifier 70, voltage comparator network 72, and output frequencydivider 74.

Positive constant current source 64 is used to supply a charging currentto capacitor 60 and includes Zener diode Z1 (type 1N827A) the cathode ofwhich is coupled to supply voltage at +10 v.d.c. The anode of diode iscoupled to one side of grounded resistor R1 (1.3K ohms) and to terminal5 of comparator 76 as a reference voltage.

The output from comparator 76 is connected to the base of NPN transistorT1 (type 2N3906) the collector of which is connected to comparatorterminal 6 and to one side of current limiting resistor R2 (1.0M ohms),the other side of which is connected to supply voltage. With thisarrangement, the emitter of transistor T1 provides a constant currentoutput at about 6 micro-amps.

Negative constant current source 66 is similarly configured and includesZener diode Z2 (type 1N827A), the anode of which is connected tonegative supply voltage at -10 v.d.c. The cathode of diode Z1 isconnected to one side of grounded resistor R3 (1.3K ohms) and toterminal 5 of comparator 78 (type MC1458). The output of comparator 78is connected to the base of PNP transistor T2 (type 2N3904), the emitterof which is connected to terminal 6 of comparator 78, and to one side ofresistor R4 (1.0M ohms), the other side of which is connected tonegative supply voltage. With this configuration, the collector oftransistor T2 provides a constant current sink, also at 6 micro-amps.

Switching circuit 68 functions to alternately switch current sources 64and 66 to alternately charge and discharge capacitor 60. Switchingcircuit 68 includes positive switching comparator 80 (type MC 1458)which receives a reference input at terminal 3 from the juncture ofresistor R5 (150K ohms), the other side of which is connected topositive supply voltage, and of resistor R6 (220K ohms), the other sideof which is connected to negative supply voltage. Resistors R5 and R6thereby form a voltage divider network to supply reference voltage tocomparator 80. The other input at terminal 2 of comparator 80 isreceived by way of current limiting resistor R7 (20K ohms) from voltagecomparator network 72. The output of comparator 80 is connected to thecathode of diode D1, the anode of which is connected to the emitter oftransistor T1, and also to the anode of diode D2.

When the input voltage at terminal 2 of comparator 80 is logic low (0volts) the output therefrom is logic high at +10 v.d.c. which reversebiases diode D1 and diverts the positive current flow through diode D2,the cathode of which is connected to capacitor 60, in order to chargecapacitor 60. When the input to comparator terminal 2 is high, theoutput terminal sinks current and thereby diverts the positive constantcurrent from current source 64 through diode D1.

Switching circuit 68 also includes negative switching comparator 82which receives a reference voltage at terminal 3 thereof in common withthe reference voltage on comparator 80. Comparator 82 receives feedbackat terminal 2 thereof from network 72 by way of resistor R8 (20K ohms).The output from comparator 82 is connected to the anode of diode D3, thecathode of which is connected to the collector of transistor T2, and tothe cathode of diode D4. The anode of diode D4 is connected to capacitor60 and to the cathode of diode D2.

When the feedback voltage at terminal 2 of comparator 82 is logic low,the output from comparator 82 is logic high at +10 v.d.c. whichforwardly biases diode D3 to provide current to negative source 66 inorder to allow capacitor 60 to charge.

When comparator terminal 2 is logic high, the output of comparator 82 islogic low at -10 v.d.c. which reverse biases diode D3 and allows source66 to sink current and thereby discharge capacitor 60 by way of diodeD4.

With this arrangement, switching circuit 68 alternately switches currentsources 64 and 66 to repetitively charge and discharge capacitor 60according to the feedback from voltage comparator network 72. Thisfeedback, as explained further hereinbelow, toggles between logic highat +10 v.d.c. and logic low at 0 v.d.c.

Capacitor 60 is also connected to and provides input to isolationcomparator 70 (type LM310) which isolates comparator 60 from voltagecomparator network 72.

Voltage comparator network 72 is configured to sense the voltage oncapacitor 60 by way of isolation comparator 70 and, specifically, isdesigned to toggle the voltage on capacitor 60 between +0.4 v.d.c. and+4.0 v.d.c. The time frequency for this to occur depends upon thecapacitance of capacitor 60 and thereby is a determination of thecapacitance.

The output from comparator 70 is connected to one side of resistor R9(1.2K ohms), the other side of which is connected to one side ofcapacitor C1 (500 p.F.), to one side of resistor R10 (91K ohms), and toterminal 2 of high toggle voltage comparator 84 (type LM311). Referencevoltage to terminal 3 of comparator 84 is supplied by the voltagedivider network composed of series connected resistors R11 (22K ohms),potentiometer resistor R12 (2K ohms full range), R13 (4.7K ohms), andR14 (270 ohms). As shown, one side of resistor R11 is connected topositive supply voltage and one side of resistor R14 is connected toground. Reference voltage from the voltage tap of resistor R12 issupplied to terminal 3 of comparator 84. The output from comparator 84is connected to the other sides of resistor R10 and capacitor C1 to bothinputs of NAND 86, and to one side of pull up resistor RP1 (22K ohms),the other side of which is connected to supply voltage at +10 v.d.c.

The output from isolation comparator 70 is also connected to one side ofcurrent limiting resistor R15 (1.2K ohms), the other side of whichsupplies input voltage to terminal 2 of low toggle comparator 88, to oneside of capacitor C2 (500 p.F.), and to one side of resistor R16 (91Kohms).

Reference voltage to terminal 3 of comparator 88 is supplied from thejuncture of resistors R13 and R14. The output from comparator 88 isconnected to the other sides of capacitor C2 and resistor R16, to oneside of pull up resistor RP2 (22K ohms), the other side of which isconnected to positive supply voltage, and to the input of NAND 90.

The output from NAND 90 is provided as one input to NAND 92, the outputof which supplies the other input to NAND 90 and also supplies thetoggle switching feedback to switching comparators 80 and 92 by way ofresistors R7 and R8 respectively. The other input to NAND 92 is suppliedfrom NAND 86.

Upon start-up, the inputs to terminals 2 of comparators 80 and 82 arelogic low. As a result, comparator 80 provides an output at +10 v.d.c.to reverse bias diode D1 which in turn diverts positive constant currentto capacitor 60 for charging. At the same time, comparator 82 providesan output at +10 v.d.c. to forward bias diode D3 and thereby supplynegative constant current to circuit 66. During the voltage rise oncapacitor 60, the output from high toggle comparator 84 is logic low andthe output from low toggle comparator 88 is logic high.

When the voltage on capacitor 60 reaches approximately 4.0 v.d.c., hightoggle comparator 84 switches state to logic high which is inverted byNAND 86 to provide a logic low signal to NAND 92 which changes state toprovide a logic high output to switching circuit 68. The outputs fromswitching comparators 80, 82 then switch state to -10 v.d.c. Comparator80 then sinks current through diode D1 from positive current source 64.The output from comparator 82 provides reverse bias to diode D4 whichallows capacitor 60 to discharge through diode D4 to negative currentsource 66.

When the voltage on capacitor 60 drops to about +0.4 v.d.c., the outputfrom low toggle comparator 88 switches state to provide a logic lowoutput to NAND 90 which toggles NAND 92 to change state to provide anoutput at logic low. This allows positive current source 64 to againsupply current by way of diode D2 to capacitor 60. Thus, the output fromvoltage comparator network 72 is a square wave which toggles between 0and +10 v.d.c.

The output from network 72 is also provided to series connectedresistors R18 and R19 (both 11K ohms). One side resistor R19 isconnected to ground as shown, and the juncture between resistors R18,R19 is connected to input terminal 10 of frequency divider 74 (typeCD4020). Frequency divider 74 divides the input frequency by a factor of2048 and provides a square wave output which toggles between 0 and +5v.d.c. at output terminal 62. Thus, the output at terminal 62 is acapacitance signal in the form of a square wave, the frequency of whichis correlated with the capacitance of capacitor 60, that is, of thecapacitance between electrodes 24 and 26 which varies as a function ofthe amount of lachrymal fluid present in the sample pad therebetween.

FIG. 4 illustrates signal processing circuit 56 which includes amicroprocessor 94 (type CDP1802), erasable, programmableread-only-memory 96 (EPSON, type 2716), address latch 98 (type CD40175),and random-access-memory 100 (RAM, type CDP1824), among othercomponents. Microprocessor 94 receives supply power at +5 v.d.c. at pin40. Ground is connected to pin 20. Input clock signals are received atterminal 1 at 1 megahertz which is provided by oscillator circuit 102.This circuit includes NAND 104, the output of which is connected to theinput of NAND 106 and to one side of crystal 108 (one megahertz). Theoutput from NAND 106 is connected to one side of grounded capacitor C4(500 p.F.) and to one side of resistor R22 (1.2K ohms). The other sideof resistor R22 is connected to the other side of crystal 108 and to theinput of NAND 110. The output from NAND 110 is connected to the input ofNAND 104 and to terminal 1 of microprocessor 94. As so configured,oscillator circuit 102 provides the required clock signals at 1megahertz.

Microprocessor 94 receives a zeroing input at terminal EFI which isconnected to the juncture between resistor R24 (22K ohms) and normallyopen zero switch 40 (see also FIG. 2). The other side of resistor R24 isconnected to supply voltage, and the other side of switch 40 isconnected to ground as shown. Switch 40 is used to provide a "zero"indication to microprocessor 94 when an unused sample pad is placedbetween electrodes 24, 26. This provides a zero level which compensatesfor relative humidity and for variations between the batches of samplepads 12.

Input to microprocessor terminal EF3 is a signal indicating that asample pad is present between electrodes 24, 26. The signal is producedby providing a light emitting diode (LED) 112 and a light receivingdiode 114 respectively disposed on opposed sides of receiving space 52.When a sample pad is present between electrodes 24, 26, the light outputfrom LED 112 is blocked and diode 114 no longer conducts. This allows avoltage rise at the anode thereof from supply voltage by way of pull-upresistor R26 (22K ohms). Power is supplied to the anode of LED 112 byway of current limiting resistor R28 (270 ohms). The cathodes of bothdiodes are connected to ground as shown.

Microprocessor 94 also receives inputs at terminal 2 thereof fromSchmitt trigger 116 the input of which is connected to ground and atterminal 3 from the output of Schmitt trigger 118. The input to trigger118 is connected to the juncture between capacitor C6 (1.0 u.F.) theother side of which is connected to +10 v.d.c. and resistor R30 (11Kohms), the other side of which is connected to ground.

Eight line date bias 120 includes lines D0-D7 connected tomicroprocessor terminals 15-8 respectively, EPROM terminals 9-17respectively, RAM terminals 14-18 respectively, and to output terminalsdesignated as 122.

Eight line address bus 124 includes lines A0-A7 connected tomicroprocessor terminals 25-32 respectively and EPROM terminals 8-1respectively. Address lines A0-A3 are connected to latch terminals 5,12, 13, and 4 respectively. As shown, additional address lines A10, 9,and 8 respectively interconnect latch terminals 15, 10, and 7 with EPROMterminals 19, 22, and 23. Additionally, lines A0-A4 interconnect EPROM96 with RAM terminals 5-1 respectively.

Microprocessor 94 also provides an output at terminal TPA to terminal 9of latch 98. Latch 98 terminal 2 is connected to two of the invertinginput terminals of NAND 126, the output of which is connected by way ofinvertor 128 to EPROM terminal 18.

Microprocessor terminal MRD is connected to AND 126, RAM terminal 16,NAND 130, to NAND 132, and to NAND 134. Microprocessor terminal MWR isconnected to RAM terminal 17.

Terminal N0 of microprocessor 94 provides an output by way of invertor136 to NAND 130. Another output is provided at terminal TPB by way ofinvertor 138 to NAND 130, NAND 132, and NAND 134. Microprocessor outputterminal N1 is connected by way of invertor 140 to NAND 132 and terminalN2 is provided as all three inputs to NAND 142, the output of which isconnected to NAND 134.

The output from NAND 134 is connected to terminal 4 of beeper controlunit 144 (type CD4098). Terminal 2 of unit 144 is connected to one sideof resistor R32 (1M ohms), the other side of which is connected tosupply voltage at +5 v.d.c., and to one side of capacitor C6 (1 u.f.),the other side of which is connected to unit terminal 1. Control unitterminal 13 is connected to ground as shown. Control unit 144 providesan output at terminal 6 thereof by way of resistor R34 (5K ohms) to thebase of transistor T3. The emitter of transistor T3 is connected toground and the collector is connected to one side of beeper 146 (Mallorybrand Minilert), the other side of which is connected to supply voltageat +10 v.d.c.

Processing circuit 56 receives the squarewave input at terminal 62 fromcapacitance circuit 54 which is provided at input to microprocessorterminal EP2. In response, as explained further hereinbelow inconnection with the operating program, processing circuit 56 producesrespective outputs at data output terminals 122, output terminal 148connected to the output of NAND 130, and output terminal 150 provided asthe output from NAND 132. These terminals present the output fromcontrol circuit 56 to display circuit 58 (FIG. 5).

In operation, a logic low output at microprocessor terminal MRD andlogic high outputs at microprocessor terminals N0 and TPB provide alogic high output at terminal 148 which enables writing data, by way ofdata bus 120 at terminal 122, for the two most significant digits ofdisplay circuit 58.

A logic low signal at microprocessor terminal MWR and logic high signalsat terminals TPB and N1 result in a logic high signal at output terminal150 which enables data writing by way of output terminal 122 for theleast significant digit of the display.

In order to activate beeper 146, microprocessor 94 produces a logic lowsignal at terminal MRD to AND 134, a logic high signal at terminal TPBby way of invertor 138 to NAND 134, and a logic law signal at terminalN2 to NAND 142 and then to NAND 134. With these three inputs at NAND134, its output goes logic low to beeper control unit 144 whichactivates unit output terminal 6 by way of resistor R34 to the base oftransistor T3 which then conducts to energize beeper 146.

FIG. 5 is an electrical schematic illustration of display circuit 58including display 42 which is supplied by least significant bitlatch/decoder 152, latch/decoder 154, and most significant bitlatch/decoder 156, all three of which are type CD4056. Display circuit58 receives display data over data bus 120 by way of terminal 122 ofwhich data lines D0-D3 are connected to decoders 152 and 156, and datalines D4-D7 are connected to decoder 154 at the terminals shown.Decoders 152 and 154 receive logic high enabling signals at respectiveterminals 1 from processing circuit 56 by way of terminal 148.Similarly, decoder 156 receives an enable signal at terminal 1 thereofby way of terminal 150.

Oscillator circuit 158 is configured to provide squarewave signals at100 hertz in order to alternate the electric field applied to the liquidcrystals at that frequency as conventionally required in LCD displays.Oscillator circuit 158 includes NAND 160 the input to which is connectedto one side of resistor R36 (47K ohms) and the output of which isprovided as input to NAND 162. The output from NAND 162 is connected toone side of capacitor C8 (0.1 u.F.) and as input to NAND 164. The outputfrom NAND 164 provides the required 100 hertz squarewave signals and isconnected to respective terminals 6 of decoders 152-156, to terminals 1and 40 of display 42, and to one side of resistor R38 (47K ohms). Theother side of resistor R38 is connected to the other sides of capacitorC8 and resistor R36.

In operation, an enable signal by way of terminal 148 to latch/decoders152 and 154 enables them to latch the data on data bus 120. Similarly,an enable signal at terminal 150 enables latch/decoder to latch the dataon data lines D0-3. The latch data is then decoded and displayed to therespective segments of display 42 by way of the lines and terminalsshown in FIG. 5.

FIG. 6 is a computer program flowchart illustrating the operatingprogram 600 for operating assessment device 20 and in particular, foroperating microprocessor 94. The program is stored in EPROM 96 and ispreferably written in machine code.

The program enters at step 602 which initializes the hardware andsoftware variables on power up. The program then moves to step 604 whichsets the variable "C" equal to zero and clears a software "zero" flag.

The program then moves to step 606 which reads the input atmicroprocessor terminal EF3 (FIG. 4) to determine if a sample pad ispresent between electrodes 24 and 26. That is to say, the presence ofsample pad which blocks the light transmission from diode 112 to diode114 results in a logic high input at terminal EFS indicative of thepresence of a sample pad. The program loops through step 606 in astandby mode until a sample is present.

When a sample is present, the answer in step 606 is yes and the programmoves to step 608 which asks whether zero switch 40 is closed. If theanswer in step 608 is yes, indicating that the zero switch is closed,the program moves to step 610 which sets the "zero" flag. If the answerin step 608 is no, the program moves to step 612 to decrement atwenty-five second delay counter which ensures that capacitance circuit54 has time to stabilize before a reading is taken. After decrementingthe delay counter one unit, the program moves to step 614 which askswhether the delay is complete, that is, whether the delay counter isdecremented to zero. If no, the program loops back to step 608 andcontinues to loop through steps 612, 614 until the delay is complete.

When the delay is complete in step 614, or after step 610, the programmoves to step 616 which asks whether the input to microprocessorterminal EF2 from capacitance circuit 54 is logic low. If no, theprogram continues to loop through step 616 until the answer is yes, atwhich point the program moves to step 618 which asks whether the inputfrom capacitor circuit 54 is logic high. If no, the program continues toloop through step 618 until the answer is yes. Steps 616 and 618 ensurethat the program marks the beginning of the logic high portion of theoutput squarewave provided from capacitance circuit 54 to microprocessorcircuit 56.

When the answer in step 618 is yes, indicating the beginning portion ofthe logic high signal, the program moves to step 620 which incrementscounter "C" after which the program moves to step 622 which asks whetherthe squarewave signal from capacitance circuit 54 has changed state tologic low. If no, the program loops back to step 620 to again incrementcounter C. The program continues to loop through step 620 and 622 at themicroprocessor clock rate as long as the capacitance circuit squarewavesignal is high. In this way, counter "C" increments to a valuerepresentative of the time length of the positive portion of thesquarewave signal. This in turn provides a value representative of thecapacitance between electrodes 24 and 26. If a sample pad placed betweenelectrodes 24 and 26 is a "dry" pad representing a "zero" level, thenthe final value of counter "C" represents the zero value.

When the input of the squarewave signal finally changes state to logiclow, the program moves to step 624 which asks whether the zero flag isset, which may have been set in step 610. If yes, indicating that a drysample pad for calibration purposes is in place, the program moves tostep 626 to store the value of counter "C" as zero value "Z". This valueis stored in RAM 100. The program then moves to step 628 to output all"zeroes" to display circuit 58.

If the answer in step 624 is no, the program moves to step 630 to storethe value of counter "C" as then existing, and to retrieve thepreviously stored zero value "Z". The program then moves to step 622 tocalculate the difference between the current counter "C" value and thezero "Z" value accordingly to the formula as shown to produce the value"V".

The program then moves to step 634 to retrieve from memory theappropriate data "D" for the value of "V" calculated in step 632. Inother words, a look-up table is stored in memory in order to produce anoutput in the form of data "D" for display which is correlated with thevalue of "V", which in turn is correlated with the value of capacitancebetween electrodes 24, 26, and which is further correlated with theamount of lachrymal fluid present in the sample pad being assessed.

The digits displayed on display panel 42 in response to data "D" areappropriately chosen to be meaningful to the operator of assessmentdevice 20. For example, the displayed digits could represent astatistical variation from the norm as empirically statistically derivedby sampling a large number of patients. For example, a display of 50would indicate a statistically normal amount of lachrymal fluid presentin a sample pad. Similarly, a reading of 95 might indicate a percentileof excessive lachrymal fluid. That is to say, data "D" would produce anoutput reading of 95 when the value "V" indicates a quantitive lachrymalfluid greater than 95% of the data base samples. Similarly, data "D"might be chosen to display "5" when the value of "V" indicates thesample a very minimal amount of lachrymal fluid as the fifth percentile.

After step 634, the program moves to step 636 to output the appropriatedata by way of data bus 120 and the appropriate enabling signals by wayof terminals 148 or 150 to display circuit 58 in order to produce adisplay in accordance therewith. The program then moves to step 638 toactivate beeper 146 for a predetermined amount of time in order to alertthe operator of assessment device 20 that the analysis is complete. Theprogram then moves to step 640 which asks whether a sample is stillpresent between electrodes 24, 26. If yes, the program continues to loopthrough step 640 until the sample which has already been analyzed isremoved. When the answer in step 640 is no, the program loops back tostep 604 and is ready for the next sample.

As those skilled in the art will appreciate, the present inventionencompasses many variations in the exemplary and preferred embodimentdescribed herein. For example, sample pad 12 can present a rectangularshape or geometrical shape other than the preferred circular.Additionally, while the assessment device is preferably configured todetermine the capacitance of a sample pad, the sample pad also presentsother parameters, such as resistance, which can be determined and theinformation processed electronically. As a further example, controlcircuit 36 could be configured totally in hardware such as with thesemiconductor chip without using a microprocessor and associatedprogram, although such is preferred for economy and flexibility in thechange of operation. Finally, while the preferred output of theassessment device is the form of a digital display using liquidcrystals, other outputs may be used such as analog meters.

Having thus described the preferred embodiments of the present invention, the following is claimed as new and desired to be secured by Letters Patent:
 1. A method of sampling lachrymal fluid coverage on the surface of a person's eye, said method comprising the steps of:providing a plurality of substantially identical lachrymal fluid sampling devices, each includinga pad composed of a material having known absorbency of lachrymal fluid and exhibiting variations in electrical capacitance according to the content of lachrymal fluid absorbed thereby, said pad having a predetermined volume and presenting a sampling face having a predetermined surface area thereby providing predetermined absorbency of lachrymal fluid when said face is in contact with the surface of a person's eye, said pad being dimensionally configured for placement completely between the electrodes of an assessment apparatus having an oscillation circuit in which the capacitance of said pad, when placed between the electrodes, forms a part of the circuit and in which the oscillation frequency thereof varies in accordance with the capacitance of said pad, the apparatus being calibrated for the dimensions of said pad and a predetermined range of lachrymal fluid contents thereof, said sampling device further including handle means coupled with said pad without enclosing any significant surface portion thereof and composed of material substantially non-absorbent of lachrymal fluid for grasping by user for placing said sampling face into contact with a person's eye and for placing said pad completely between the electrodes of the assessment apparatus; grasping one of said sampling devices by said handle means and placing said sampling face into engagement with the surface of the person'eye; removing said sample pad from the surface of the person's eye after a predetermined time, said pad thereby containing a representative sample of the lachrymal fluid content of the person's eye; and placing said pad between the electrodes of the assessment device and determining the oscillation circuit frequency thereof, such being representative of the pad capacitance and thereby representative of the lachrymal fluid coverage on the surface of the person's eye.
 2. The method as set forth in claim 1, further including the step of composing said sample pad of polyvinylformal.
 3. The method as set forth in claim 1, further including the step of providing said sample face with a surface area between about 25 and 30 square millimeters.
 4. The method as set forth in claim 1, further including the step of providing said sample pad with a disk-shaped configuration.
 5. The method as set forth in claim 1, further including the step of providing said sample pad with a pore size between about 30 and 150 microns.
 6. The method as set forth in claim 5, further including the step of providing said sample pad with a pore size of about 60 microns.
 7. The method as set forth in claim 1, further including the step of providing said sample pad with a thickness of about 1 millimeter.
 8. The method as set forth in claim 1, further including the step of composing said handle of synthetic resin material.
 9. The method as set forth in claim 8, further including the step of composing said handle of mylar.
 10. The method as set forth in claim 1, said removing step including the step of removing said sample pad after about three and five seconds.
 11. The method as set forth in claim 1, further including the steps ofincluding a detachable sample pad cover over said sample pad for protecting said sample pad from contamination, and detaching said cover from said sample pad before placement thereof in an absorbent relationship with a person's eye.
 12. A device for sampling lachrymal fluid coverage on the surface of a person's eye comprising:a lachrymal fluid sampling pad composed of a material having known absorbency of lachrymal fluid and exhibiting variations in electrical capacitance according to the content of lachrymal fluid absorbed thereby, said pad having a predetermined volume and presenting a sampling face having a predetermined surface area thereby providing predetermined absorbency of lachrymal fluid when said face is in contact with the surface of a person's eye, said pad being dimensionally configured for placement completely between the electrodes of an assessment apparatus having an oscillation circuit in which the capacitance of said pad, when placed between the electrodes, forms a part of the circuit and in which the oscillation frequency thereof varies in accordance with the capacitance of said pad; and handle means coupled with said pad without enclosing any significant surface portion thereof and composed of material substantially non-absorbent of lachrymal fluid for grasping by user for placing said sampling face into contact with a person's eye and for placing said pad completely between the electrodes of the assessment apparatus.
 13. The sampling device as set forth in claim 12, said sample pad being composed of polyvinylformal.
 14. The sampling device as set forth in claim 12, said sample pad presenting a surface area between about 25 and 30 square millimeters.
 15. The sampling device as set forth in claim 12, said sample pad presenting a disk-shaped configuration.
 16. The sampling device as set forth in claim 12, said sample pad presenting a pore size between about 30 and 150 microns.
 17. The sampling device as set forth in claim 16, said sample pad presenting a pore size of about 60 microns.
 18. The sampling device as set forth in claim 12, said sample pad presenting a thickness of about 1 millimeter.
 19. The sampling device as set forth in claim 12, said handle means including an elongated strip composed of synthetic resin material.
 20. The sampling device as set forth in claim 19, said handle means being composed of mylar.
 21. The sampling device as set forth in claim 12, said sample pad being operable for absorbing a representative sample of lachrymal fluid after contact with a person's eye for about 3 to 5 seconds.
 22. The sampling device as set forth in claim 12, further including a detachable cover for covering said sampling pad for preventing contamination thereof before use. 